Recovery of hydrocarbons from subsurface reservoirs involves the drilling of one or more wells to the depth of the hydrocarbon reservoir. After well completion, the reservoir can be drained for hydrocarbon fluids that are transported to the surface.
The reservoir typically has different zones with different permeability. If the permeability of one zone is higher than the average permeability in the reservoir it is sometimes referred to as a “thief zone”.
A thief zone is a permeable zone adjacent to a less permeable zone. Typically, a thief zone is classified as a zone that has higher than average permeability compared to the rest of the reservoir. A thief zone may be a naturally occurring region of the reservoir, such as layers and fractures formed during the natural geologic process. Thief zones may also be man made, such as wormholes created by acid injection (acid is often injected in an attempt to increase hydraulic conductivity in enhanced oil recovery, see for example Rege & Fogler, AIChE Journal 35(7), 1989, 1177-1185, and Singurindy & Berkowitz, Water Resources Research 39(1), 2003, 8.1-8.14; Emmanuel & Berkowitz, Advances in Water Research 28, 2005, 337-344; Singurindy & Berkowitz, Advances in Water Research 28, 2005, 507-521; that cites Rege & Fogler), or induced fracturing (such as described in e.g. WO 02/095188 A1).
Thief zones in the form of streaks of high permeability are common in carbonate reservoirs and can speed up, and increase the risk of the production well producing large volumes of water, if such thief zone connects the production well to a source of water.
Thief zones are also detrimental to the sweep efficiency, when fluid is injected into the reservoir from injection wells to displace oil. If the reservoir is compromised by a thief zone connecting the injection well with the production well, then the production well will produce large amounts of the injected fluid. Such short-circuiting in the reservoir further bypasses considerable amounts of hydrocarbons, as the thief zone will prevent an efficient sweep of the area beyond the thief zone. Thus, the existence of thief zones can cause inefficient production of hydrocarbons from a reservoir, and further reduce the recovery of hydrocarbons.
Consequently, much effort has gone into developing methods to and products that reduce the permeability of thief zones.
Many of these have attempted to reduce permeability either at the well face or within a few feet of the bore by using e.g. cement squeezes. When the thief zone is only sealed near the well, the flow behaviour of the thief zone will continue just beyond the sealed region as if no sealing had occurred. As the typical thief zone is not confined to a few feet, but rather may extend over a large distance from the well; measures that seal the thief zone near the well are of limited value.
Time delayed gelling and in situ polymerization methods have been previously attempted in the field to plug thief zones. These methods typically suffer from difficulty of placing the monomer and catalyst the desired place, and from the complexity of achieving the correct timing. Another problem associated with time delayed gelling and in situ polymerization methods are the fact that an injected fluid front will expand radially from the injection site. Consequently, this can result in the gel and polymer sealing other areas than the thief zone, which is also detrimental to sweep efficiency.
U.S. Pat. No. 4,809,780 to Shen describes a method for selectively sealing thief zones in a formation penetrated by an injection well and a production well. This is done by injecting a heat-sensitive sealing fluid into the injection well. As shown in FIG. 1 the injected fluid preferentially travels in the high permeability zone. However, it also travels into the low permeability zone. The injection of sealing fluid is followed by injection of a push water of high salinity. Induction of electrical resistance heating preferentially heats up the thief zones. Since most of the sealing fluid passes through the thief zone they are selectively heated, and consequently sealed. The selectivity of this method is dependent on timing and the sealing not having significantly diffused into the adjacent lower permeable zones before heating is initiated. Consequently, the method appears likely to suffer from at least some of the drawbacks mentioned for the time delayed gelling and in situ polymerization methods.
SU 775 294 A1 discloses a method for isolating water-saturated formations adjacent to oil and gas deposits using a sealing solution containing waste from alkylation of paraffins in the presence of concentrated sulphuric acid. Effectiveness of the sealing of low-permeable formations at a temperature above 100° C. is enhanced by using alkylated sulphuric acid as a sealing agent. Alkylated sulphuric acid can be obtained as the waste from alkylation of paraffinic hydrocarbons with olefins in the presence of concentrated sulphuric acid as a catalyst. Calcium compounds in the water-bearing formations reacts with sulphuric acid passing insoluble barrier. The process takes about 5 h. During this time the shaft should be closed.
This document does not teach to use an acid without a content of hydrocarbons.
SU 1 709 071 discloses a method for plugging up wells bottom zone by interaction of lime suspension and clay solution preliminarily pretreated and containing water-stable compounds e.g. aluminium sulphate. The wells fluid is substituted by a clay solution which has been treated with a chemical reagent forming with the solutions calcium oxide compounds which are stable in water e.g. Al2(SO4)3, Fe2(SO4)2, Na2SiF6. Then the bottom hole zone is subjected to action of streams of suspension containing lime. The streams drive the clay solution into the seam where part of it is coagulated due to presence of Ca-ions and compact it. The excess of lime forms with the clay hydrates increasing effeciveness of the screen.
Hence, there is a need for a method for sealing a thief zone that provides for a simple mode of operation. A need further exists for a method that, when injected into a subsurface formation, will substantially fill a thief zone. A need further exists for a method for sealing a high permeability zone that extends a considerable distance from the well. A need further exists for a method that, after setting in a high permeability zone, provides resistance to flow at pressure gradients experienced within the reservoir during normal operations.